Warehouses are facing an extended assortment and diversity of products, with more orders for small units. Typically, all of this has to be organized out of the same warehouse. An important part of warehousing goods is picking of products for sale, which generally is done manually, because the products, due to their sizes, need to be stored at different locations and consquently also need to be retrieved from different locations. This results in a time and labor consuming process of order picking. The costs of order picking can represent 55% of operational warehousing costs.